1. Field of the Invention
The present invention relates to spectrophotometric methods and spectrophotometers.
2. Description of the Prior Art
For determining the concentration of a specific substance, it is known to generate radiation, whose intensity is modulated to a defined degree of modulation, which passes through the specific substance. A measurement signal for a radiation wavelength at which the specific substances absorbs the radiation is generated by a first detector, and a reference signal for a radiation wavelength, other than the specific substance's radiation wavelength, is generated by a second detector and the concentration of the specific substance is determined with a signal analyzer from the measurement signal and the reference signal.
European Application 0 196 993 describes a known spectrophotometer operating in this manner. The known spectrophotometer is a gas concentration measuring apparatus which measures the concentration of a specific, non-elementary polyatomic gas in a gas mixture. The gas passes through a cuvette which is irradiated by a pulsating source of radiation. The radiation then passes a first interference filter which is transparent to a wavelength at which the specific gas absorbs radiation and strikes a first detector which then generates a measurement signal. Next to the first interference filter is arranged a second interference filter which is transparent to a wavelength at which the specific gas does not absorb radiation. The radiation which passes the second interference filter strikes a second detector which then generates a reference signal. The measurement signal and the reference signal are amplified and filtered in respective capacitors, the alternating current part of the measurement signal and the reference signal being filtered out. Each signal is synchronously rectified, whereupon the ratio between the two signals is formed. The ratio signal then constitutes a normalized measurement signal which is independent of detector temperature (provided the detectors change temperature uniformly), the aging of the radiation source, etc. The normalized measurement signal only depends on the concentration which can thereby be calculated.
The source of radiation in the known spectrophotometer pulsates at a frequency of primarily 1-30 Hz. Since the source of radiation is pulsed, measurement results are not affected by interference, such as radiation leakage from other sources acting on the detectors etc. The radiation consists of an alternating intensity component superimposed on a fixed intensity component, so that the detectors thereby generate an alternating current component and a direct current component. The direct current component is eliminated in the capacitor, but since both components in the measurement signal depend to the same extent on the concentration of the specific substance, rapid fluctuations in concentration can result in the direct current component passing the capacitor, thereby producing an erroneous value for the concentration. In addition, the response time for changes in concentration is dependent on the frequency used. The most rapid response in the known spectrophotometer is attained at 30 Hz. An increase in the frequency of the known spectrophotometer would reduce the modulation of, and hence variations in, the source of radiation, thereby reducing the alternating current component. As a result, errors caused by the passage of the direct current component through the capacitor when there are rapid fluctuations in concentration would, in turn, be larger, and the signal-to-noise ratio of the direct current component would be impaired, since the direct current component would decline at higher frequencies.